1. Field of the Invention
The present invention relates to a generator set having an inverter and more particularly, to a generator set having an inverter where a generator of particularly a magnetic type can be operated at high efficiency within a wider speed range.
2. Description of the Related Art
It has increasingly been common that an engine operated generator for use as an alternating current power source is equipped with an inverter for stabilizing the output frequency. For example, such a generator set having an inverter is disclosed in Japanese Patent Laid-open Publication (Heisei)11-308896 where an alternating current power is generated by an engine operated generator, converted into a direct current form, and converted again by the inverter into an alternating current form of a commercial frequency. Because its frequency does not depend on the speed of the engine, the output of the generator set having an inverter can be adjusted to a desired power output which is preset or determined to match the magnitude of load by controlling the speed of the engine.
FIG. 9 illustrates a power output characteristic of a generator of a magnetic type where the speed is a parameter. As shown, the curves A, B, and C represent the relationship between the voltage and the current of the output when the [revolution] speed of the generator is expressed by H, M, and L (H greater than M greater than L), respectively. The curves Ap, Bp, and Cp represent the outputs of the generator determined by the curves A, B, and C, respectively. The direct current voltage at the input of the inverter can thus be controlled to a target level V for maintaining the inverter output to match the load.
As apparent from the drawing, with the direct current voltage at the input of the inverter maintained at the target level V, the outputs of the generator at L, M, and H of the revolution are expressed by the points pxe2x80x2, qxe2x80x2, and rxe2x80x2 respectively designated on the corresponding curves Ap, Bp, and Cp which are defined by the intersections between A and V, between B and V, and between C and V, respectively. The power output is almost a maximum level at M of the speed while is a pre-low level at L of the speed and a post-low level at H of the speed. More particularly, the speed should stay in a narrow range about M for allowing the generator to produce generally a maximum of the output or operate at an optimum efficiency.
When the speed of the engine is controlled for adjusting the output of the generator, it should be operated within a no-voltage-shortage range, e.g., as shown in FIG. 9, where the voltage is equal to or higher than a target level V. On the other hand, if the speed is too high, the power output will be reduced as a result of preferable utilization of the over-current side of the characteristic.
It is, hence, an object of the present invention to provide a generator set having an inverter which can be operated in a wider speed range while eliminating the foregoing drawbacks.
This allows the target voltage level to be set to a lower value at a lower range of the speed of the magnetic generator and a higher value at a higher range of the same. Accordingly, as the magnetic generator is increased in the voltage output substantially in proportion to the revolution, it can produce a voltage output of the target level at the lower range of the revolution. Also, its voltage output can be close to the maximum level without lowering the efficiency when the generator is operated at the higher range of the revolution. As a result, the useful range of the speed of the generator can be increased.
As a second feature of the present invention, a generator set having an inverter may further comprise an engine revolution controlling means for controlling the revolution of an engine to drive the generator so that the conduction rate of the semiconductor rectifying devices is converged on a predetermined target rate, wherein the controlling of the speed of the engine is implemented by adjusting the supply of fuel to the engine.
This allows the speed of the engine to be modified within a wider range thus controlling the conduction rate of the semiconductor rectifying devices to a desired level.
As a third feature of the present invention, the engine revolution controlling means may be arranged to decrease the speed of the engine when the deviation of the conduction rate from the target rate is positive and increase the same when negative. As a fourth feature of the present invention, a rate of change of revolution of the engine is greater at an increase than at a decrease thereof.